Dual spectra optical pyrometers are well known in the art and have been used to measure the temperature of turbine blades in an operating jet engine. The radiation or light from the turbine blade is comprised of an emitted component from the blade itself and a reflected component from a fireball within the jet engine. The presence of a reflected component within the light from the turbine blade causes the pyrometer to provide an erroneously high indication of blade temperature.
In the dual spectra optical pyrometer disclosed by Gebhart, et al in U.S. Pat. No. 4,222,663, light from a turbine blade is provided to two pyrometers having different spectral bands whose outputs are subsequently processed to provide an estimate of the magnitude of the reflected energy. Since silicon is typically used for the detectors, an optical filter whose passband is a portion of that of silicon must be positioned in the optical path of one of the pyrometers to generate the needed spectral band difference. To provide an acceptable signal to noise ratio, prior art dual spectra optical pyrometers have spectral bands comprising 0.4 to 0.85 microns for the filtered pyrometer and 0.4 to 1.1 microns for the unfiltered one.
The radiation from the turbine blade must be gathered by the pyrometer and provided to a pair of detectors which converts the optical energy into an electrical signal for subsequent processing. To guide the light from the turbine blade, split it, and provide it to each detector, dual spectra optical pyrometers of the prior art typically use a bifurcated optical fiber bundle. Since less energy is available at lower temperatures overall performance is limited. Consequently, the use of bifurcated optical fiber bundles results in lowered accuracy because of the lower signal to noise ratios for lower temperatures associated therewith. Bifurcated optical fiber bundles are burdened with high production costs as well as high optical losses. Moreover, the durability of bifurcated optical bundles is much less than that of a single optical fiber.